Magnetic recording head having a corrosion-protective layer of a metal salt of a perfluorinated polyether acid

ABSTRACT

A process is provided for synthesizing metal salts of perfluorinated polyethers containing at least one carboxylic acid group. The polymeric salts so provided are effective as anti-wetting and corrosion-protective agents. The metal salts of perfluorinated polyether acids may be used to prepare corrosion-protected substrates, including magnetic recording disks and magnetic recording heads.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 09/759,117,filed Jan. 11, 2001, now U.S. Pat. No. 6,638,622 B2; the disclosure ofwhich is incorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates generally to perfluorinated polyethers and usesthereof. More particularly, the invention pertains to metal salts ofperfluorinated polyethers having carboxylic acid end groups, to methodsfor synthesizing the metal salts, and to use thereof as anti-wettingand/or corrosion-protective agents, e.g., in anti-wetting and/orcorrosion-protective coatings on metal-containing substrates,particularly in magnetic recording devices such as magnetic recordingdisks and magnetic recording heads.

BACKGROUND

It is well known that organic carboxylic acids, R—COOH, in a fluid state(liquid or vapor) primarily exist in the hydrogen bonded dimeric form,as follows:

It is also well known that the vapor phase in equilibrium with solidNaCl at elevated temperature (T≧500° C.) is dominated by the dimericspecies

The formation and stability of the latter are attributed to thepair-wise Coulombic interaction of the constituent ions; see Doan et al.(1997) J. Am. Chem. Soc. 119:9810. The heat of dimerization for theacetic acid dimer has been measured to be 15 kcal/mol (Taylor (1951) J.Am. Chem. Soc. 73:315; Weltner (1955) J. Am. Chem. Soc. 77:3941), andthat for (NaCl)₂ to be 48 kcal/mol. The corresponding pair-wiseinteraction between salt molecules of carboxylic acids, e.g., sodiumacetate, has only recently been reported, by Doan et al. (1997), supra.

Perfluoropolyethers (PFPEs) are currently in use as lubricants in avariety of high-performance applications. PFPEs are commerciallyavailable in several distinct structural forms. Representative PFPEs areknown by the brand names Demnum® (Daikin Kogyo Co., Ltd., Japan),Krytox® (DuPont Specialty Chemicals, Deepwater, N.J.), and Fomblin® Z(Zentek SRL, Milan, Italy), having the following structural formulae

Krytox is synthesized by base-catalyzed polymerization ofhexafluoropropylene oxide, as described by Gumbrecht (1966) ASLE Trans.9:24, while Demnum is made similarly but starting with2,2,3,3-tetrafluorooxetane. The hydrogen atoms in the resulting polymersare replaced by fluorine atoms by subsequent contact with F₂ insolution, as described by Ohsaka (1985) Petrotech (Tokyo) 8:840. FomblinZ is synthesized by photooxidation of tetrafluoroethylene and is alinear, random copolymer of ethylene oxide and methylene oxide units;see Sianesi (1973) Chim. Ind. 55:208.

These PFPEs are also available with carboxylic acid end groups, asexemplified by Fomblin® Z-DIAC (Zentek SRL, Milan, Italy), Krytox®-H(DuPont) and Demnum®-SH (Daikin Kogyo Co. Ltd., Japan), having thestructures

respectively. It has been found that the sodium salts of these polymersnormally exist in the dimeric form under ambient conditions; see Doan etal. (1997), cited supra.

The inventors herein have now discovered that these and other metalsalts of perfluorinated polyethers having one or more carboxylic acidgroups are extremely effective anti-wetting agents and thus find utilityin a host of applications, for example in corrosion-protective films.Although Doan et al. describes a method for synthesizing sodium salts ofPFPE acids, the method described is problematic. That is, Doan et al.describes preparation of sodium salts of PFPE acids by reacting a PFPEacid (e.g., Fomblin® Z-DIAC, Krytox-H® or Demnum®-SH) with a sodiumhydroxide solution, and then extracting the salt from the resultingemulsion with a fluorocarbon solvent. Although the intended product maybe prepared using this technique, the method requires handling amultilayer fluid including a viscous interfacial gel layer, a cumbersomeprocess that requires extreme care. This invention is in part directedto a new method for synthesizing metal salts of perfluorinated polyetheracids that overcomes the aforementioned disadvantage of the Doan et al.synthesis.

The invention is also premised on the discovery that metal salts of PFPEacids are useful as anti-wetting agents and corrosion-protective agents.In this regard, it should be pointed out that certain fluorinatedpolymers, particularly poly(fluoroalkylacrylates) andpoly(fluoroalkylmethacrylates), have been used as oil and waterrepellent agents (see, for example, B. E. Smart, “Organic FluorineCompounds” in Kirk-Othmer Encyclopedia of Chemical Technology, ThirdEdition, Vol. 10, John Wiley & Sons, New York, 1980, p. 869). However,the adhesion force obtained using these polymers as coating agents isinsufficient to provide sufficient durability in many contexts.

One important application of the present compounds that exploits thenewly discovered properties is as corrosion-protective agents that bondstrongly to metal and metal oxide substrates, as the compounds adherewell to metal-containing substrates. Furthermore, the compounds of theinvention are extremely useful as corrosion-protective agents formagnetic recording disks and magnetic recording heads, particularlythose having a carbon overcoat. Such overcoats are typically formed bysputter deposition from a graphite target, and are generally calledprotective carbon overcoats, “diamondlike” carbon overcoats, amorphouscarbon overcoats, or, in the case of those overcoats formed by sputterdeposition in the presence of a hydrogen-containing gas, hydrogenatedcarbon overcoats. Tsai et at. in “Structure and Properties of SputteredCarbon Overcoats on Rigid Magnetic Media Disks,” J. Vac. ScienceTechnology A6(4), July/August 1988, pp. 2307-2314, describe suchprotective carbon overcoats and refer to them as amorphous “diamondlike”carbon films, the “diamondlike” referring to their hardness rather thantheir crystalline structure. IBM's U.S. Pat. No. 4,778,582 describes aprotective hydrogenated disk carbon overcoat formed by sputtering agraphite target in the presence of Ar and hydrogen (H₂). The carbonovercoats may also be formed by plasma-enhanced chemical vapordeposition (CVD) and may include nitrogen in addition to hydrogen, asdescribed by Kaufman et al. (1989) Phys. Rev. B 39:13053.

To increase the areal density of the data magnetically recorded on thedisk, the recording head must be brought close to the magnetic layer,which means that the overcoat thickness must be substantially reduced,i.e., to less than 5 nm in future disk drives. Consequently, animportant challenge faced by the disk drive industry is how to makeprotective disk overcoats that are ultra-thin yet still provide thedesired durability and corrosion protection. However, the carbonovercoat sputter-deposited on the magnetic layer of storage disks oftenabounds with pinholes, through which the corrosion of metals in themagnetic and other underlayers may occur. Reducing the thickness of thecarbon overcoat exacerbates the problem. The same drawback isencountered with the metallic elements of magnetic recording heads thatare coated with a layer of sputtered carbon. Because metal salts of PFPEacids adhere well to metal-containing surfaces and poorly to carbonovercoats, the compounds are able to fill pinholes in the protectiveovercoat without adding any substantial thickness to the disk.Anti-corrosion strength is further enhanced by the exceptional waterrepellency of PFPE acid salts.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to address theabove-mentioned need in the art by providing a method for synthesizingmetal salts of PFPE acids.

It is another object of the invention to provide such a method whichinvolves treating a perfluorinated polyether having at least onecarboxylic acid group with a metal salt of a volatile organic acid, andthen volatilizing the resulting organic acid.

It is another object of the invention to provide a method for treating ametal-containing substrate to enhance water repellency and corrosionresistance, wherein the method involves depositing a compositioncontaining a metal salt of a perfluorinated polyether acid onto thesubstrate.

It is an additional object of the invention to provide such a methodwherein the metal-containing substrate has a metal surface or a surfacecomprised of a metal oxide.

It is a further object of the invention to provide a corrosion-protectedmagnetic recording disk comprised of a substrate, a magnetic layer, andan amorphous carbon overcoat on the magnetic layer that has been treatedwith a composition containing a metal salt of a perfluorinated polyetheracid as a corrosion-protective agent.

It is yet a further object of the invention to provide acorrosion-protected magnetic recording head having an amorphous carbonovercoat that has been treated with a composition containing a metalsalt of a perfluorinated polyether acid.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a cross-section of a magnetic recordingdisk treated with a corrosion-protective composition containing a metalsalt of a PFPE acid as a corrosion-protective agent.

FIG. 2 schematically illustrates a cross section of a treated substratewherein PFPE acid salts as provided herein fill the pinholes present ina sputtered carbon layer.

FIG. 3 shows the IR spectra of Fomblin Z-DIAC and its sodium salt, asdescribed in Example 1.

FIG. 4 shows the IR spectra of Demnum-SH and its sodium salt, also asdescribed in Example 1.

FIG. 5 shows the IR spectra of the sodium salt of Demnum-SH before(upper spectrum) and after (lower spectrum) rinsing withtrifluoroethanol (TFE), as described in Example 2.

FIG. 6 shows the IR spectra of the sodium salt of Fomblin Z-DIAC before(upper spectrum) and after (lower spectrum) rinsing withtrifluoroethanol (TFE), as described in Example 2.

FIG. 7 shows the IR spectra of the sodium salt of Demnum-SH adsorbedonto an aluminum plate (upper spectrum) and onto a substrate surfacecomposed of a nickel-iron (NiFe) alloy, as described in Example 3.

FIG. 8 shows the IR spectra of the sodium salt of Fomblin Z-DIAC presentas a film on a bare aluminum plate (upper spectrum) and on an aluminumplate coated with a carbon film (lower spectrum), as described inExample 4.

FIG. 9 shows the microscopic images of corrosion debris developed bycerium etching of the magnetic disks prepared in Example 4. The upperimage illustrates the results obtained with the bare carbon-coatedmagnetic disks, while the lower image illustrates the results obtainedwith the disks treated with a corrosion-protective compositioncontaining a metal salt of a perfluoropolyether acid as described inExample 5.

FIG. 10 shows the microscopic images of corrosion debris developed byexposing to an atmosphere equilibrated with 1 N HCl solution (a) anuntreated magnetic recording head assembly having an amorphous carbonovercoat (left side), (b) a magnetic recording head assembly having anamorphous carbon overcoat treated with a corrosion-protectivecomposition containing the sodium salt of Demnum-SH (upper right), and(c) a magnetic recording head assembly having an amorphous carbonovercoat treated with a corrosion-protective composition containing thesodium salt of Fomblin Z-DIAC (lower right), as described in Example 6.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the present invention in detail, it is to beunderstood that this invention is not limited to specific process steps,substrates, magnetic recording devices, or the like, as such may vary.It is also to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto be limiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “and,” and “the” include pluralreferents unless the context clearly dictates otherwise. For example, “asalt” refers to a mixture of salts as well as a single salt, “a polymer”(e.g., “a PFPE”) refers to a mixture of polymers (e.g., PFPEs) as wellas a single polymer (e.g., a PFPE), and the like.

The compounds to which the present invention is addressed are metalsalts of perfluorinated polyethers having at least one carboxylic acidend group. These salts are also referred to herein as “metal salts of aPFPE acid,” as “PFPE acid salts” and as compounds having the formula“PFPE-COO⁻M⁺”(wherein M is the metal atom of the salt), it beingunderstood that the perfluorinated polyether PFPE can contain more thanone —COO⁻M⁺ moiety and possibly some degree of hydrogenation as well. Anovel synthesis for preparing such salts is provided, which, as notedabove, overcomes the disadvantages inherent in the prior art. The saltsare synthesized by treating a perfluorinated polyether having at leastone carboxylic acid end group with a metal salt of a volatile organicacid under reaction conditions effective to convert the carboxylic acidgroup(s) of the perfluorinated polyether acid to the salt form andvolatilize the resulting organic acid, resulting in a reaction productcomprising a metal salt of the PFPE acid. The perfluorinated polyetherthat serves as the starting material is comprised of monomer unitshaving the structure —CF₂—O—, —CF₂—CF₂—O—, —CF₂—CF₂—CF₂—O—, —CF(CF₃)—O—,—CF(CF₃)—CF₂O—, or a combination thereof, and has at least onecarboxylic acid group, generally at one terminus of a substantiallylinear polymer, and typically has two carboxylic acid groups, onepresent at each terminus of a substantially linear polymer. Suchpolymers include, but are not limited to, the commercially availablepolymers Fomblin® Z-DIAC, Krytox®-H and Demnum®-SH. Although referred toherein as a “perfluorinated” polyether, the polymer may be partiallyhydrogenated, in which case up to about 50% of the fluorine atoms in theperfluorinated polyether are substituted with a hydrogen atom. Thepolymeric acid that serves as the starting material in the syntheticprocess generally has a number average molecular weight in the range ofapproximately 500 to 10,000, preferably 1000 to 5000, most preferably2500 to 3500.

The metal salt of the volatile organic acid has the structure RCOO⁻M⁺where M is the metal, preferably an alkali metal, e.g., sodium, and R ishydrocarbyl, typically alkyl, and preferably lower alkyl. Thus, anexemplary alkali metal salt is sodium acetate, CH₃COONa. The presentmethod exploits the fact that the organic acid component of the alkalimetal salt is volatile; for example, acetic acid has a boiling point of118° C. As noted above, the perfluorinated polyether starting materialis reacted with the metal salt of the volatile organic acid underreaction conditions effective to convert carboxylic acid groups to thesalt form and volatilize the resulting organic acid. Suitable reactionconditions involve heating a mixture of the perfluorinated polyetheracid and the metal salt of the volatile organic acid at a temperature ofat least about 130° C. for at least 48 hours. Preferably, a fluorinatedsolvent such as perfluorohexane (e.g., FC72® 3M, St. Paul, Minn.) isthen added to the reaction mixture and the mixture is heated at refluxuntil smooth. Additional fluorinated solvent may be added at this point;in addition, a lower alkanol such as methanol may be added to removeunreacted metal salt, and trifluoroethanol may be added to minimize gelformation. The reaction product is then isolated by extraction, e.g.,using a lower alkanol such as methanol. Specific examples describingsynthesis of the sodium salt of Demnum-SH and Z-DIAC, respectively, areincluded herein as Examples 1 and 2.

These PFPE acid salts may be applied to a substrate surface to form afilm thereon. Alternatively, a pinhole-containing layer on asubstrate—e.g., an amorphous carbon coating on a metallic substrate—maybe treated with a solution of a PFPE acid salt such that the pinholesbecome filled with the salt solution, preventing corrosion of theunderlying metal-containing layer or layers. The compounds prepared assynthesized above have been found to be extremely effectivewater-repellent and oil-repellent agents, and adhere well tometal-containing surfaces, e.g., metallic surfaces or surfaces of metaloxides. Adhesion to an aluminum surface is described in Example 3, andadhesion to a NiFe alloy surface is described in Example 4.

One important area in which the present compounds find utility is ascorrosion-protective layers in magnetic storage devices such as magneticstorage disks and magnetic recording heads. Of particular interest aremagnetic storage disks and magnetic recording heads that have anovercoat of essentially amorphous carbon, as disclosed, for example, inU.S. Pat. No. 5,030,494 to Ahlert et al. and U.S. Pat. No. 5,075,287 toDoerner et al., both assigned to IBM Corporation. As explained in theaforementioned patents, many rotating rigid disk drives includeread/write transducers (or “heads”) supported on a carrier (or “slider”)that ride on a cushion or gearing of air above the surface of a magneticrecording disk when the disk is rotating at operating speed. The sliderhas an air-bearing surface (“ABS”), typically in the form of a pluralityof rails, and is connected to a linear or rotary actuator by means of asuspension. There may be a stack of disks in the disk drive with theactuator supporting a number of sliders. The actuator moves the slidersradially so that each head may access the recording area of itsassociated disk surface. The slider in the disk drive is biased towardthe disk surface by a small force from the suspension. The ABS of theslider is thus in contact with the disk surface from the time the diskdrive is turned on until the disk reaches a speed sufficient to causethe slider to ride on the air bearing. The ABS of the slider is again incontact with the disk surface when the disk drive is turned off and therotational speed of the disk falls below that necessary to create theair bearing. This type of disk drive is called a contact start/stop(CSS) disk drive. To provide wear resistance for the ABS in a CSS diskdrive, a protective carbon overcoat may be placed on the slider rails.IBM's U.S. Pat. No. 5,159,508 describes a slider with air-bearing railshaving an amorphous carbon overcoat that is adhered to the rails by asilicon adhesion layer.

The magnetic recording disk in a CSS rigid disk drive is typically athin film disk comprising a substrate, such as a disk blank made ofglass, ceramic, glassy carbon or an aluminum-magnesium (AlMg) alloy witha nickel-phosphorous (NiP) surface coating, and a cobalt-based magneticalloy film formed by sputter deposition over the substrate. A protectiveovercoat, such as a sputter-deposited amorphous carbon film, is formedover the magnetic layer to provide corrosion resistance and wearresistance from the ABS of the slider. The overcoat may further includerelatively small amounts of embedded iron (Fe), tungsten (W) or tungstencarbide (WC) to improve wear resistance and minimize the likelihood ofdamage to disk file components (see U.S. Pat. No. 5,030,494 to Ahlert etal., cited above). Such overcoats are typically formed by sputterdeposition from a graphite target, and as explained in the Backgroundsection, are generally called protective carbon overcoats, “diamondlike”carbon overcoats, amorphous carbon overcoats, or, in the case of thoseovercoats formed by sputter deposition in the presence of ahydrogen-containing gas, hydrogenated carbon overcoats. In addition tothe magnetic layer and the protective overcoat, the thin film disk mayalso include a sputter-deposited underlayer, such as a layer of chromium(Cr) or a chromium-vanadium (CrV) alloy, between the substrate and themagnetic layer, and a sputter-deposited adhesion layer, such as a Cr,tungsten (W) or titanium (Ti) layer, between the magnetic layer and theprotective overcoat.

As alluded to above, a problem associated with the aforementionedprotective overcoat is that the sputter-deposited carbon layer oftenabounds with pinholes, through which the corrosion of metals in themagnetic and other underlayers may occur. These pinholes can be detectedin a number of ways, for example using a cerium etching technique, whichemploys (NH₄)₂Ce(NO₃)₆ as an oxidizing agent and selectively oxidizesmetal, e.g. chromium metal, in the underlayer. Corrosion debris evolvedat pinhole sites may thus be observed and counted with an opticalmicroscope. The present compounds, since they adhere well to metal andmetal oxide surfaces but do not adhere to amorphous carbon, can beapplied to the surface of the amorphous carbon overcoat so as to fill inthe pinholes and eliminate or at the very least minimize the possibilitythat the underlying layers may corrode.

Thus, in one embodiment, an improved magnetic recording disk is providedthat is fabricated by treatment with a metal salt of a perfluorinatedpolyether acid. At a minimum, the magnetic recording disk comprises: asubstrate; a magnetic layer formed over the substrate; an overcoatformed over the magnetic layer, the overcoat being a film having asubstantially planar surface and comprising primarily carbon in theessentially amorphous form and optionally one or more of tungsten ortungsten carbide distributed throughout and embedded within the carbon;and a corrosion-protective composition filling pinholes in the carbonovercoat, the corrosion-protective composition containing a metal saltof a PFPE acid as a corrosion-protective agent. Optimally, the magneticlayer is comprised of a cobalt-based magnetic alloy film formed bysputter deposition over the substrate; a particularly preferredcobalt-based magnetic alloy is CoPtCrB, as described in IBM's U.S. Pat.No. 5,523,173 to Doerner et al.

A thin film disk 10 according to the present invention is illustrated insection in FIG. 1. The disk 10 includes a substrate 12, typicallycomprising a disk blank made of glass, ceramic, glassy carbon or analuminum-magnesium (Al—Mg) alloy with a nickel-phosphorous (Ni—P)surface coating. A chromium (Cr) or a chromium-vanadium (Cr—V) alloy 14underlayer is sputter-deposited on the substrate. Over the underlayer 14is deposited a magnetic layer 16, which preferably, as explained above,is comprised of a cobalt-based magnetic alloy such as CoPtCrB. Over themagnetic layer 16 is overcoat 18 of sputter-deposited amorphous carbon,containing pinholes 20. The pinholes 20 are filled with acorrosion-protective composition 22 containing a PFPE acid salt; fillingthe pinholes in this way prevents exposure of the underlying magneticlayer. Magnetic recording head 24 is mounted on arm 26, which isconnected to means (not shown) for positioning head 24 in a generallyradial direction with respect to disk 10. FIG. 2 illustrates morespecifically how molecules of the PFPE acid salt 28, with polar, ionicend group 30, fill the pinholes 20 in the amorphous carbon overcoat 18,protecting areas 32 in the underlying metal-containing magnetic layer 16that would otherwise be susceptible to corrosion. It may be desirable tothen coat the pinhole-filled surface with a layer of aperfluoropolyether lubricant (not shown).

As noted above, the PFPE metal salts of the invention are also useful inproviding corrosion protection for magnetic recording heads andassociated head assembly components, particularly when asputter-deposited amorphous carbon overcoat is used as described withrespect to magnetic recording disks. A magnetic recording head assemblyof the invention will generally include at least one component comprisedof an oxide, nitride or carbide of aluminum, zirconium, silicon ortitanium and have an overcoat of sputter-deposited amorphous carbon,wherein the improvement lies in the use of a corrosion-protective agentof the invention to fill pinholes present in the carbon overcoat andprevent exposure of the underlying layer.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties.

EXPERIMENTAL

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toprepare and use the oligomers and polymers disclosed and claimed herein.Efforts have been made to ensure accuracy with respect to numbers (e.g.,quantities, temperature, etc.) but some errors and deviations should beaccounted for. Unless indicated otherwise, parts are parts by weight,temperature is in ° C. and pressure is at or near atmospheric.Additionally, all starting materials were obtained commercially orsynthesized using known procedures.

EXAMPLE 1

Synthesis of Metal Salts of PFPE Acids:

Demnum-SH: The sodium salt of Demnum-SH was prepared as follows.Twenty-five (25) grams of Demnum-SH (Daikin Kogyo Co., Ltd., Japan) and2 grams of sodium acetate were mixed in a 100 ml beaker. The mixture wasstirred at 140˜150° C. with stirring. The mixture bubbled and becamerigid within about an hour. Heating at 145° C. was continued for threedays until bubbling stopped completely. The reaction mixture was thentransferred into an 250 cc Erlenmeyer flask, and 50 cc of FC72(perfluorohexane) were added. A condenser was attached, and the mixturewas refluxed at 60° C. until smooth. The mixture was then transferredinto a 250 cc separatory funnel, and 50 cc of FC72 (additional), 50 ccof methanol (for removal of excess Na-acetate), and 10 cc of TFE(trifluoroethanol, CF₃—CH₂—OH) were added to minimize gel formation. Themixture was shaken in the separatory funnel vigorously and then allowedto stand overnight. Two clear layers developed. The upper methanol layercontained excess sodium acetate. The lower layer (˜100 cc) contained thedesired Demnum-SH salt. The lower layer was collected in a deepcontainer, and heated gently (˜50° C.) overnight to remove the solvent.The product was then dried in a vacuum oven maintained at ˜100° C.overnight.

The sodium salt of Fomblin Z-DIAC (obtained from Ausimont USA, Inc.) wasprepared following essentially the same procedure. In this case,however, after the separatory funnel process, only ˜50% (by weight) ofthe expected product was recovered from the FC72 layer. The balance ofthe material was found in the CH₃OH layer. An IR analysis proved thatboth fractions were the Na salt of Fomblin Z-DIAC. An F-19 NMR analysisrevealed, however, that the number average molecular weight of the CH₃OHfraction was 1500, while that of the FC72 fraction was 3000. The numberaveraged molecular weight of the starting Fomblin Z-DIAC was 2000. Themolecular weight of the starting Demnum-SH was 3000. Since Demnum-SH ismonofunctional, while Z-DIAC is bifunctional, it was concluded that inthe synthesis of the Na salt of Demnum-SH, very little product moleculesresulted that were small enough to be soluble in methanol. It wasfurther concluded that, for the Na salts of Fomblin Z-DIAC of molecularweight ˜1500, the polarity rendered by the Na-carboxylate groupsattached at both ends of a molecule is such that it makes the entiremolecule soluble in methanol.

FIG. 3 shows the IR spectra of the starting Fomblin Z-DIAC and its Nasalt of molecular weight 3000. FIG. 4 shows the IR spectra of startingDemnum-SH and its Na salt. In both cases the product integrity isattested by the complete shift of the carbonyl band from ˜1780 to ˜1680cm⁻¹; see Doan et al. (1997) J. Am. Chem. Soc. 119:9810.

For simplicity, the abbreviations Z(COONa)₂ and D-COONa will be usedhereinafter to indicate respectively the Na salts of Fomblin Z-DIAC andDemnum-SH thus synthesized. Unless mentioned otherwise, Z(COONa)₂ refersto the product obtained in the FC72 fraction (with the number averagedmolecular weight of ˜3000), as does D-COONa (also with the numberaveraged molecular weight of ˜3000).

It was found that both Z(COONa)₂ and D-COONa were sparingly soluble inFC72 (perfluorohexane; b.p. 58° C.) but were quite soluble in TFE(trifluoroethanol, b.p. 77° C.) and also in HFE-A(nonafluoro(iso)butyl-methyl ether; b.p. 60° C.).

EXAMPLE 2

Adhesion of Z(COONa)₂ and D-COONa on an Aluminum Surface:

FIG. 5 shows the IR spectra (obtained by the reflectance technique) ofZ(COONa)₂ applied as a thin smear on an aluminum plate (upper spectrum)and after the same plate was rinsed thoroughly with pure TFE (lowerspectrum). The corresponding spectra observed with D-COONa are shown inFIG. 6. The spectra observed after the TFE rinse were attributed to amolecular monolayer of the salt molecules adhering to the aluminumsurface. Essentially the same spectrum was observed from an aluminumcoupon which was dipped into a TFE solution of Z(COONa)₂ (0.3 wt. %) andthen rinsed thoroughly with pure TFE.

A further study revealed that these monolayers of Z(COONa)₂ or D-COONaadhering to the aluminum surface were not removed when the samplecoupons were immersed in water with stirring for one hour, nor when theywere immersed in TFE with stirring for one hour.

The thickness of the D-COONa film remaining on an aluminum plate afterthe dip-and-rinse process was measured by the ellipsometry technique anddetermined to be 70 Å. The contact angle (of water droplet) of thealuminum plate increased from 43° to 115° upon adhesion of D-COONa bythe dip-and-rinse process. The contact angle determined for the treatedplate was close to that of a Teflon-coated surface. It is thus stronglysuggested that adhesion occurs solely due to the interaction between thepolar sodium-carboxylate unit and the polar metal surface, with thenonpolar perfluoropolyether moiety extending freely outward.

As expected, the aluminum surface thus treated was extremely waterrepellent and oil repellent. The treated surface does not permit writingwith either a water-based pen or an oil-based pen.

EXAMPLE 3

Adsorption on Other Metal Surfaces:

In order to demonstrate that the observed strong adsorption of the Nasalts of PFPE-acids was not uniquely limited to the aluminum surface asdescribed in Example 2, adsorption of D-COONa upon a NiFe alloy surfacewas examined. FIG. 7 compares the spectra observed from an aluminumplate and a NiFe disk, both of which had been dipped into a TFE solutionof D-COONa (0.3 wt. %) and then thoroughly rinsed with pure TFE. It wasrevealed that a substantially identical quantity of D-COONa moleculesadhered to the NiFe surface.

EXAMPLE 4

Interaction Between Na-PFPE Carboxylate and Sputtered Carbon:

In order to verify that the observed strong adsorption of D-COONa orZ(COONa)₂ to metal surfaces was due to the Coulombic interaction betweenthe extremely polar (ionic) Na-carboxylate end group of the polymermolecule and the polar constituents of the metal (oxide) surface, theadsorption of Z(COONa)₂ upon aluminum plates coated with a carbon filmof thickness ˜200 A was evaluated. The IR spectra observed from bare andcarbon coated aluminum plates which had been soaked in a TFE solution ofZ(COONa)₂ (1%) for three hours followed by thorough rinsing with pureTFE are compared in FIG. 8. The immersion process was used in order toensure (possible) bonding to a porous carbon medium. It was revealedthat only a trace amount of D-COONa was adsorbed on the carbon coatedaluminum. The ellipsometry measurement showed the presence of Z(COONa)₂corresponding to a thickness of 5 Å. It was surmised that the smallamount of adsorption occurred through pinholes of the carbon film.

EXAMPLE 5

Sealing Pinholes of Carbon Overcoat on Magnetic Storage Disk:

Since Na-PFPE carboxylate molecules adhere strongly to a metal oxidesurface but not to the surface of sputtered carbon, Na-PFPE carboxylatemolecules adhere to the metal oxide layer of a magnetic storage diskthrough pinholes in a sputter-deposited amorphous carbon overcoat. FIG.9 compares the microscopic images of corrosion debris developed by thecerium etching technique on two disks of the same lot. The ceriumetching technique uses (NH₄)₂Ce(NO₃)₆ as an oxidizing agent andselectively oxidizes the underlayer. Corrosion debris evolved at pinholesites may thus be observed and counted with an optical microscope. Priorto etching, one of the disks was dipped into a TFE solution of D-COONa(1%) followed by a thorough rinsing with pure TFE. It was revealed thatthe pinhole density was reduced by a factor of three by thedip-and-rinse process of a D-COONa/TFE solution.

EXAMPLE 6

Corrosion Protection of GMR Heads:

It is particularly essential that the metallic elements of GMR (giantmagnetoresistance) heads in magnetic storage disk systems are protectedfrom corrosion. To that end, an entire head assembly was coated with alayer of sputtered carbon. The corrosion, however, could still developthrough pinholes of the carbon overcoat. It was found that by exposingan HGA (the head and gimbals assembly) to an atmosphere equilibratedwith 1.0 N HCl solution for thirty minutes, a sufficient amount ofcorrosion debris developed that was observable by an optical microscope.FIG. 10 shows, on the left side, the images of several heads that havebeen subjected to this accelerated HCl vapor test. Shown on the rightside are the images of heads that had been first immersed in a TFEsolution of D-COONa (1%) or Z(COONa)₂ (1%), rinsed sequentially withpure TFE, FC72, and IPA (isopropyl alcohol), and then subjected to theHCl vapor test. The efficacy of the immersion in a Na-PFPE carboxylate,solution is evident. We surmise, as before, that pin holes in the carbonlayer are effectively sealed by Na-PFPE carboxylate molecules, andhydrophobic PFPE chains extending outward block intrusion of polarmolecules such as H₂O and HCl.

1. In a magnetic recording head comprised of (a) a substrate of anoxide, nitride or carbide of aluminum, zirconium, silicon or titaniumand having (b) an overcoat comprising a film composed primarily ofcarbon in the essentially amorphous form and having pinholes thereinexposing the substrate, wherein the improvement comprises treating themagnetic recording head with a corrosion-protective composition so as tofill the pinholes of the overcoat, the composition containing acorrosion-protective agent comprised of a metal salt of a fluorinatedpolyether having at least one carboxylic acid group.
 2. The magneticrecording head of claim 1, wherein the fluorinated polyether is aperfluorinated polyether.
 3. The magnetic recording head of claim 2,wherein the perfluorinated polyether has one carboxylic acid end group.4. The magnetic recording head of claim 3, wherein the perfluorinatedpolyether has two carboxylic acid groups.
 5. The magnetic recording headof claim 2, wherein the perfluorinated polyether is comprised of monomerunits having the structure —CF₂—O—, —CF₂—CF₂—O—, —CF(CF₃)—O—,—CF(CF₃)—CF₂—O—, or a combination thereof.
 6. The magnetic recordinghead of claim 1, wherein the fluorinated polyether is a linear polymer.7. The magnetic recording head of claim 1, wherein the metal salt is analkali metal salt.
 8. The magnetic recording head of claim 1, whereinthe fluorinated polyether has a number average molecular weight in therange of approximately 500 to 10,000.
 9. The magnetic recording head ofclaim 8, wherein the fluorinated polyether has a number averagemolecular weight in the range of approximately 1000 to
 5000. 10. Themagnetic recording head of claim 9, wherein the fluorinated polyetherhas a number average molecular weight in the range of approximately 2500to
 3500. 11. The magnetic recording head of claim 1, wherein theimprovement further comprises coating the carbon overcoat with alubricating film of a perfluoropolyether prior to deposition of thecorrosion-protective composition.